Thanks, BDub! That really helps to know – and makes sense after watching endless debug output.
Reuben @Hologram has a compatible explanation – the SIM may have had data written to its blacklist at some point. https://community.hologram.io/t/hologram-sim-not-working-on-particle-boron/2507/8?u=michaelm
I’ll try to get you the AT+CGDCONT= value as soon as I can get the NB 1500 back online. Right now, none of my formerly fine-working Hologram SIM’s will connect with the Arduino NB1500. I let them “rest” all weekend, too – completely powered down.